1. Field of the Invention
This invention relates to electron beam particle systems and more particularly to an imaging section of a charged particle beam probe forming or projection system, such as an Electron Beam Projection System (EBPS).
2. Description of Related Art
Stray fields extending from lenses in close proximity to a charged particle beam column can cause aberrations or other disturbances; and an imbalance of fields at a critical portion of a column can distort the beam or its path greatly reducing the quality of performance of the system.
In electron beam (E/B) systems incorporating large area reduction projection optics, it is critical to minimize electron-optical performance detractors, such as aberrations and positional disturbances.
Accordingly there is a need for a way to compensate for those factors.
In some cases, use of a conventional lenses to compensate for these problems is not possible because of limitations of space in a column of electromagnetic lenses where space is at a premium.
Where a moving image plane is involved, the sum of all fields from one, two or three lenses should be zero. If not then the result will be aberrations and/or positional disturbances which as pointed out above are unsatisfactory.
An object of this invention is to provide a lens in a charged particle beam system which can null fields thereby providing compensation of unwanted fields to provide fields with a sum of zero field at the image plane.
The term xe2x80x9cdoubletxe2x80x9d as used herein denotes a pair of lenses operated under a specific symmetry condition, established in the following way:
A source (of particles) illuminates an object in front of a lens pair. The object is located precisely in the front focal plane of the first lens. The first lens generates an image of the source between the pair, and an image of the (closer) object at infinity. This effectively collimates the rays of particles emerging from the object. Accordingly the first lens is labeled xe2x80x9ccollimatorxe2x80x9d. The second lens is positioned exactly such that its front focal plane coincides with the back focal plane of the first lens. The second lens focuses the collimated, therefore parallel rays at its back focal plane, which then becomes the image plane for the object. Since the object is now projected into the image plane, the second lens is refined to as xe2x80x9cprojectorxe2x80x9d. Under this condition the optical magnification of the lens pair is given by the ratio of the focal lengths of projector to collimator, M=f2/f1. Simultaneously, the objectxe2x80x94image distance, is given by L=2(f1+f2). If lenses of the same shape are used, i.e. if they are by mathematical definition xe2x80x9csimilarxe2x80x9d, then their sizes scale with their respective focal lengths. For example, if f1=4f2, the collimator must be four times as large as the projector to maintain similarity of the lenses, Consequently, the point of coincidence on the axis of focal points between the lenses, located at a distance from the object of z1=2f1, and of z2=2f2 from the image, constitutes a point of symmetry. The doublet is often referred to as xe2x80x9csymmetricxe2x80x9d about the point of coincidence on the system axis. In the special case of f1=f2 or unity magnification, the term often used is xe2x80x9cmirror-symmetricxe2x80x9d, even though in the strict mathematical sense the doublet is xe2x80x9cpoint symmetricxe2x80x9d.
If the source is placed infinitely far upstream of the doublet, its image will appear at the coincidence or symmetry plane. As a consequence, all rays originating from any point on the source or its intermediate image at the symmetry plane will be parallelized by the projector. The doublet then is characterized as a xe2x80x9ctelecentric symmetric doubletxe2x80x9d. If the lenses are of the magnetic type, their field polarities are generally chosen to be opposite to each other as to completely cancel the image rotation caused by each individual lens. One then speaks of an xe2x80x9cantisymmetric doubletxe2x80x9d.
The reason for operating the lenses in the described way as a doublet is that several aberrations are eliminated (as one lens compensates the aberrations of the other lens in exactly the right ratio) and consequently image blur is reduced.
In the context of the present invention xe2x80x9cfine-tuningxe2x80x9d means as follows:
1(a) positioning of an (intermediate) image plane at exactly the right (axial) location, usually, where an aperture is mounted, for the purpose of maximum beam current transmission, or where a special optical plane is located such as the xe2x80x9csymmetry planexe2x80x9d of the aforementioned doublet;
1(b) nulling/bucking of the field fringes of the lenses constituting the doublet, either to control/eliminate interference of the fields of lenses adjacent to the doublet under consideration, or to minimize the axial field strength as well as the gradient at an object or image plane, to minimize the lateral motion of the (deflected) beam in response to axial shifts of any such plane; in other words, to provide optimum xe2x80x9ctelecentricity, of the lens system. With this purpose in mind, such auxiliary lenses are often referred to as xe2x80x9cbuckingxe2x80x9d lenses.
In accordance with this invention a system, apparatus and a method are provided wherein a charged particle beam system includes a source of charged particles producing a beam directed along a path including a given electromagnetic lens located along the path. The given electromagnetic lens is adapted to produce a first field directed with a first orientation adapted for affecting a beam of charged particles directed along the path through the lens. A bucking electromagnetic lens is juxtaposed with the given electromagnetic lens adapted to produce a bucking field directed with a bucking orientation adapted for affecting the beam of charged particles directed along the path, opposing the field of the given electromagnetic lens. A fringe field from the bucking electromagnetic lens produces a nulling field to minimize aberrations and disturbance of the particle beam due to target stage motion. Such disturbance can be caused by as follows:
a) eddy currents generated in metallic components in a stage moving through the magnetic field of the lens;
b) axial displacement of the image plane and consequently lateral shift of the beam landing position,
c) magnetization of magnetic inclusions of the target of substrate.
Further in accordance with this invention a first shroud is formed surrounding the given electromagnetic lens having a pair of pole pieces with the shortest distance between the pole pieces being along a direction parallel to the path. A bucking shroud surrounds the bucking electromagnetic lens having a pair of bucking pole pieces with the shortest distance between the bucking pole pieces being along a direction transverse to the path. The bucking shroud is located adjacent to the first shroud.
A further electromagnetic lens is located along the path. The further electromagnetic lens is adapted to produce a further field directed with the first orientation adapted for affecting the beam of charged particles directed along the path through the lens. The further electromagnetic lens and the given electromagnetic lens are aligned along the axis with the given electromagnetic lens following the further electromagnetic lens along the path from the source of charged particles. The further electromagnetic lens is preferably located along the axis between the auxiliary lens and the given electromagnetic lens.
Preferably, an auxiliary lens is located along the path between the source and the given electromagnetic lens with an aperture located within the auxiliary lens.
Preferably, the auxiliary lens is located at an object plane and the bucking lens is located before a following image plane.
Preferably, the lenses and the electromagnetic coils are toroidal in configuration.
Preferably, It is also a preferred alternative that the system comprises an electron beam projection system.